Recordings of single unit activity in the vestibular nucleus are monitored during horizontal head movements in cat and monkey. Neuronal mechanisms reflecting the utilization of sensory input are examined with respect to two reflex conditions: stabilization of the head on the trunk, as sensed by vestibular apparatus, and stabilization of the head on the trunk, as sensed by neck afferents. Animals are trained to perform quantifiable and repeatable eye-head movements in which these two sensory sytems are variably related to stability of the gaze during voluntary active movements or passive movements. Single neuronal units in the vestibular nuclei related to head movements are examined for their temporal correlation with onset, maintenance, and termination of head movements; for their role in processing identified afferent inputs; and where possible, for their contribution to descending spinal control of neck muscle contraction. The mechanism of gaze shifts as reflected by afferent and efferent characteristics in homologous nervous structures are analyzed on a comparative basis in the two species above, as well as in the laboratory rat, rabbit, and in bush baby (Galago senegalensis).